The invention relates to a waveguide structure with diffused rare-earth doping in a light-guide channel arranged in a lithium niobate crystal surface.
From R. Brinkmann et al, "Annealed erbium-implanted single-mode LiNbO3 waveguides", 1990, OSA Techn. Digest Series, Vol. 5, post deadline paper, PD1, an erbium-doped lithium niobate optical fibre waveguide is known which is manufactured by means of erbium ion implantation and subsequent tempering. This involves planar arrangement of the erbium doping close to the surface. The erbium ion beams destroy the crystal structure close to the surface, and this destruction has to be healed again in a separate stage of the process, so that the crystal becomes optically useful once more. So long as the healing is by means of a heat treatment, the greater mobility which they enjoy in the destroyed region means that the embedded erbium atoms prefer to migrate towards the surface of the crystal and less so into the depths, which is where the light-guide channel requires effective erbium doping. Besides, no narrow lateral doping boundary to the narrow light-guide channel is provided, and this gives rise to erbium fluorescence losses due to reabsorption in the lateral ridges and troughs of the waveguide channel, since the .sup.4 I.sub.14/2 -.sup.4 I.sub.15/2 erbium fluorescence junction is part of a three-level system and results in the ground state.